When was the last time you were enthralled by a big-budget sci-fi flick?

In a post-“Bladerunner” / “Matrix” world, it all seems so been-there-done-that and big-studio manufactured sci-fi has practically become the movie world equivalent of Microsoft: a stinking rich, Evil Empire that just encourages you to pitch those tomatoes; but at the end of the day, they continue to rule the world.

Take the case of “I, Robot”: Every frame attests to hundreds of hours of marketing meetings, thousands more hours put in by studio serfs at SFX programs and millions of dollars thrown into set and robotics design and action sequences. And the result? Well, it’s probably a good thing Dr. Isaac Asimov, the original author of “I, Robot,” has passed away, because this would have given him convulsions. Fox Pictures politely states on the title credits that the story “is suggested” by Asimov and not actually based on his book (they just happen to have the same title, that’s all!). To think that in the 1970s, Asimov had plans to adapt “I, Robot” for the big screen with aspirations for a “really adult, worthwhile science-fiction movie.”

And now here it is, 30 years later. Much of the original story has been scrapped in “I, Robot,” save for Asimov’s famed Three Laws of Robotics: 1) A robot may not injure a human being; 2) A robot must obey a human being except when it comes into conflict with the first law; and 3) A robot must protect its own existence as long as it does not conflict with the first or second laws.

It’s Chicago and the year is 2035. The Three Laws have been in effect for some time. People stroll placidly among robot sanitation workers, robot FedEx carriers, robot bartenders. Everyone thinks robots — or “canners” — are a wonderful modern convenience, and is content to live under giant electronic ads from US Robotics, soon to release a new line of dazzlingly sophisticated models called the NS-5. Unlike the robots of old, they’re much more mobile, graceful and able to simulate love and loyalty without actually feeling those emotions.

But there’s one man who can’t stand robots in whatever form: bad-boy police detective Del Spooner (Will Smith). Spooner is a maverick in more ways than one — his choice of footwear is Converse sneakers (“vintage 2004, baby”), his apartment is so retro it looks like it’s just been vacated by Elwood Blues and he listens to early 21st-century rock on a hand-operated CD player. Oh, and he likes to eat his grandmother’s homemade pie while doing his morning workout, a scene in which the camera lingers long and lovingly over his smooth biceps and customized abs.

Spooner is plagued by robot nightmares for reasons unexplained until an hour into the movie, but for now chases away haunting memories with a hot shower, standing in the tub in a way that shows off some of his other body parts. After that, we get to see him dress in snazzy leather (Neil Barrett) and slide into an Italian-type sports car to go to work. The point of this whole segment seems to be that whatever else robots have going for them, they just don’t have the bod or lifestyle of Del Spooner.

Still, the NS-5s look pretty good, especially one called Sonny (Alan Tudyk in a voice-over), whose thoughtful features in a peaked little face and lithe movements make “him” seem like a sophisticated space alien. Sonny is “unique,” designed by US Robotics’ Dr. Alfred Lanning (James Cromwell). But then the latter is found splayed on the floor of the company lobby in an apparent suicide. Del doesn’t think so and puts some hard questions to CEO Robertson (Bruce Greenwood, the perpetual Hollywood bad guy) and company psychiatrist Susan Calvin (Bridget Moynahan), whose job is to make the robots seem “more human.”

Del insists that Sonny is implicated in Lanning’s murder, but Susan argues that the Three Laws make this impossible. You know where they go from there: an exchange of semihostile one-liners (“excuse me, I’m allergic to bulls***ting”), followed by a gradual rapprochement and the hint of a budding romance. Much to the credit of director Alex Proyas (“Dark City”) though, there are no love scenes. None. NONE! This alone restored my faith in mankind and movies, a little.

The story could have used some more of this kind of (in deep voice) “respecto” or just some plain old-fashioned restraint. Proyas and his team cram hard action (including the obligatory car-chase sequence), robot ethics and politics, sci-fi hardware and an almighty, know-it-all computer called Viki (a she-version of HAL in “2001: A Space Odyssey”) into two hours.

It would take some incredible editing acrobatics to tie up all the loose ends. But in the last 20 minutes “I, Robot” gives up all pretensions of doing so and slides over to battlefield mode, where it’s man vs. robot in a rather forced volte-face of the Three Laws. By then, the robots no longer seem like robots — they’ve become an oppressed labor class clamoring for vengeance. Instead of pursuing this path, though, Proyas wraps everything up in an ending so silly and convenient it makes you want to kick the bucket. Of popcorn, that is.